CHEMICAL STUDIES ON PODZOLIC ILLUVIAL HORIZONS

Abstract

Summary In qualitative studies with a number of soils, organic reagents such as aqueous acetylacetone or cupferron were found to extract a large fraction of the organic matter from podzolic B horizons; smaller quantities were removed from two humic‐gley A horizons. All other soil types tested yielded negligible amounts of carbon with this treatment. Substitution of Cu for the exchangeable cations of a typical Australian black earth followed by treatment with acetylacetone did not result in an increase in organic matter extracted over the untreated soil. It is concluded that, to be extracted by solutions of organic chelate compounds, soil organic matter must be immobilized by ions of one of the transition metals, and the H form must be readily dispersed; it is also possible that large amounts of clay may inhibit extraction. This type of organic matter probably occurs only in the podzolic group of soils. A comparison of the extractive properties of several reagents on two Queens‐ land ground‐water podzols is presented. Cupferron is the most efficient, but complete removal of excess reagent from the final suspension is difficult. Acetylacetone (in water) and oxine (in 50 per cent. acetone) extract a large fraction of the total organic carbon and most of the Al, but the presence of acetone partially inhibits the extraction of carbon. Aqueous oxine is unsatisfactory owing to the extreme dilution of the saturated solution. It is concluded that acetylacetone is the most useful reagent so far tested owing to its miscibility with water and formation of stable ether‐soluble complexes with Fe and Al. Attempts to fractionate the B‐ horizon humus, using salicylaldehyde, acetylacetone and oxine which form progressively more stable complexes with Fe and Al, were unsuccessful owing to the dispersing effect of the NaOH added to adjust the pH values of each reagent. It is suggested that factors controlling the flocculat on of organic colloids from soil suspensions are equally as important as the bonding energy of the metal‐organic matter bonds in determining the extent of dispersion.